1. Field of the Invention
The present invention relates generally to a weft yarn sensor for use in weaving looms of an air jet shuttleless type and particularly to a weft yarn sensor for sensing whether a weft yarn has satisfactorily arrived at a predetermined position during insertion or not in weaving looms in each of which a weft yarn is inserted by employing a weft guiding comb including weft guiding members and spacers each guiding member of which has an aperture for guiding the weft yarn inserted and a gap for allowing the weft yarn to slip out of the aperture outside the guiding member and each guiding member and each spacer of which are alternately arranged and which are integrally pressed against each other by, for example, a through bolt and a nut.
2. Description of the Prior Art
As is already known in the art, a conventional weft yarn sensor of this type includes a sensor body 10 as shown in FIG. 1 of the accompanying drawings which has a form, an aperture 11 and a gap 12 about similar to those of the guiding member of the guiding comb. The sensor body 10 is provided with weft sensing means including first and second light transmitting means 13 and 14 which both are made of optical fibers such as, for example, glass fibers or clear plastic fibers and confront each other at the gap 12 of the sensor body 10 to form light projecting and receiving faces 16 and 17, respectively. The first light transmitting means 13 transmits light from a light source (not shown) to the light projecting face 16 and projects the light from the face 16 to the light receiving face 17. The second light transmitting means 14 transmits the light from the light receiving face 17 to a light receiving device (not shown) such as a phototransistor. When the weft yarn 18 passes through the gap 12 outside the aperture 11, it intercepts partially the light projected from the face 16 to the face 17.
The light projecting and receiving faces 16 and 17 both have been in the form of circles as shown in FIG. 2 of the accompanying drawings. The diameter of the circle of each of the faces 16 and 17 is considerably larger than the thickness or diameter of the weft yarn 18. For example, when the circle of each of the faces 16 and 17 has the diameter of 2 millimeter and the weft yarn 18 has the diameter of 50 denier as an example, since the maximum beam intercepted by the weft yarn 18 is merely one twentieth (1/20) of the whole beam projected to the face 17, the quantity of light transmitted from the face 17 to the light receiving device and accordingly the output of the light receiving device is slightly varied in accordance with interception and failure of interception of the light by the weft yarn 18. It has been difficult to sense such a slight change in the output of the light receiving device.
As a solution to this problem, it is considered to reduce the diameter of the circle of each of the faces 16 and 17 to a value about equal to the diameter of the weft yarn 18. However, this solution is undesirable since an excessively small quantity of light is fed to the light receiving device and the light receiving device such as a transistor is unable to respond to or be sensitive to such a fine light or a feeble light.
Furthermore, when the loom weaves fabrics of spun yarn made of short fibers such as cotton yarn, since a fly fluff is apt to be produced and it is in the form of about a circle, the fly fluff stuck to the light projecting and/or receiving faces 16 and/or 17 has intercepted the whole beam projected to the face 17 so that the conventional weft sensor has malfunctioned as if the weft yarn has been properly inserted notwithstanding that the weft yarn has not been properly inserted.